Articulated tower systems have emerged as one of the most reliable low maintenance tanker mooring systems for use in remote hostile environments. One advantage of this type of system over those which utilize a freely floating buoy held by caternary chains, is the fact that fluid-carrying conduits can be placed to extend through the rigid tower to protect them, where this is desired. Another advantage is that the tower can extend high above the water to provide an attachment point for a hawser connected to a ship, without destabilizing the system. In a typical articulated loading tower system, the rigid tower has a buoyancy compartment near the water surface, and a ballast chamber near the bottom of the tower to offset much of the buoyancy. When a moored tanker pulls the top of the tower to one side, tilting of the tower causes the buoyancy compartment to move in an arc about the pivot joint. The horizontal component of this movement times the vertically acting buoyant force results in a righting movement that tends to restore the tower to its vertical position. However, as the water depth becomes shallower, a given angle of tower tilt results in an ever decreasing moment arm which proportionately decreases the restoration force. This can be compensated for by utilizing a larger buoyancy compartment, but this results in a large and costly system.
At moderate to shallow depths, prior art articulated tower mooring systems not only have the disadvantages of requiring larger buoyancy and ballast chambers, and larger bases and pivot couplings, but also result in increased effective pivoting stiffness of the system and a consequent reduction in the tower natural period of oscillation. That is, for the ship mooring force to produce as small an angle of tower tilt as in deeper water, a stiffer tower results that tends to oscillate with a shorter oscillation period. The pivoting stiffness of the system results in more frequent breakage of hawsers connecting the tower to a ship, inasmuch as the tower applies large forces when a ship drifts slightly, rather than applying small forces over a long distance of ship drifting. The higher natural frequency of the tower is disadvantageous, because it is closer to typical wave frequencies, which tends to amplify the dynamic wave induced motion of the tower and cause large relative motions between the tower and a large relatively stable tanker. These large relative motions between tower and tanker are difficult to accommodate with conventional mooring methods. A mooring system of the articulated loading tower type, which avoided the great stiffness and large oscillation amplitude of prior art systems in water of moderately shallow depth, such as on the order of 300 feet, would be of great value.